The inhomogeneous electron spin dephasing time, T2*, is limited to ~10 ns in GaAs quantum dots due to the contact hyperfine interaction with the GaAs host nuclei.1 Due to this fast dephasing time we are exploring novel materials systems that are expected to exhibit extended spin coherence times. There are several feasible approaches for increasing spin coherence times in semiconductors. One possiblity is to fabricate quantum dot devices using isotopically pure materials containing only spin-0 nuclei. Examples are 12C and 28Si. An alternative approach, which takes advantage of the vast body of work performed on GaAs heterostructures, is to use hole spins instead of electron spins. Hole spin wavefunctions have p-like orbitals and are therefore predicted to be immune to hyperfine dephasing since the wavefunction overlap of the p-like electron orbital with the nucleus is negligible.2 Strong-spin orbit interactions resulting from the electronic structure of the valence band can be suppressed when the hole spins are tightly confined in a quantum dot. Theory suggests that quantum dot hole spin lifetimes are comparable to electron spin lifetimes.3 These predictions have recently been confirmed by measurements of hole spin lifetimes in self-assembled quantum dots using optical techniques.4,5
1. J. R. Petta et al., Science 309, 2180 (2005).
2. G. Burkard, Nature Mat. 7, 100 (2008).
3. D. V. Bulaev and D. Loss, Phys. Rev. Lett. 95, 076805 (2005).
4. D. Heiss et al., Phys. Rev. B 76, 241306 (2007).
5. B. D. Gerardot et al., Nature 451, 441 (2008).
In collaboration with Mansour Shayegan's group in Electrical Engineering, we are measuring quantum point contact and quantum dot devices fabricated from (311) GaAs hole gases. Current efforts are aimed at reducing the amount of charge switching noise found in these materials. Future efforts will focus on measurements of hole spin relaxation and coherence times.
References1. J. R. Petta et al., Science 309, 2180 (2005).
2. G. Burkard, Nature Mat. 7, 100 (2008).
3. D. V. Bulaev and D. Loss, Phys. Rev. Lett. 95, 076805 (2005).
4. D. Heiss et al., Phys. Rev. B 76, 241306 (2007).
5. B. D. Gerardot et al., Nature 451, 441 (2008).
Quantized conductance in a top-gated hole gas quantum point contact. The device was fabricated at Princeton by Javad Shabani and measured in the Petta lab dilution refrigerator.